What dust issue? All digital cameras accumulate dust on the sensor over time. There are cleaning kits available (rocket blower, the Pentax "lollipop", and fluid solutions) as well as service centres that offer cleaning (for a fee, or maybe within warranty). The Canon will also get dust on the sensor eventually. Or was there dust on there before you even started using it? In that case, an exchange would make sense.
Now, Canon is not a bad company, for sure. But recently they have been putting their focus on video, and you generally get more bang per buck from Pentax. With Canon, it kind of only makes sense if you go with the top-tier models (because Pentax isn't even competing in that range at the moment). But the good thing with Canon is that you can use a simple adapter to get Pentax lenses on it
Its a tough call. Can you get your money back and buy a K-30 (or K-50) at a different store?
K-30:
WR - get a WR lens with it and you can go out in rain. Great for travel, great if you live in a place where bad weather is common
TAv mode (great for photographers who want a little more automation than M, but still keep a lot of control over their photos)
In-body image stabilization - means it works with all lenses, even ones that predate AF
Pentax lens compatibility, lots of great prime lenses, legacy lenses. Can get great IQ without spending thousands on lenses
Great 16MP sensor, live view has focus peaking
Compact and solidly built, has convenient Pentax green button
Canon 700D:18MP instead of 16MP, might be useful for poster prints, but its an insignificant difference
Swivel screen - something I miss in Pentax models, but some people don't like it. Its subjective
Video capabilities - Canon beats Pentax in the video
Flash system - more convenient than Pentax flashes at the moment (though, Pentax has recently announced some new flashes)
Support - Canon is a bigger brand, so there are more service centres and more third party products like lenses and flashes available